This invention is concerned generally with a customized low energy method of breaking rock in a controlled manner.
As used herein the word “rock” includes rock, ore, coal, concrete and any similar hard mass, whether above or underground, which is difficult to break or fracture. It is to be understood that “rock” is to be interpreted broadly.
A number of techniques have been developed for the breaking of rock using non-explosive means. These include a carbon dioxide gas pressurisation method (referred to as the Cardox method), the use of gas injectors (the Sunburst technique), hydrofracturing and various methods by which cartridges containing energetic substances pressurise the walls or base of a sealed drill hole to produce a penetrating cone fracture (known as PCF).
These techniques may be an order of magnitude more efficient than conventional blasting in that they require approximately 1/10 of the energy to break a given amount of rock compared to conventional blasting using high explosives. The lower energy reduces the resulting quantity of fly rock and air blast and to an extent allows the rockbreaking operation to proceed on a continuous basis as opposed to the batch-type situation, which prevails with conventional blasting.
Most non-explosive rock breaking techniques rely on the generation of high gas pressures to initiate a tensile fracture at the bottom of a relatively short drill hole. If the force which is generated by the high gas pressure can be optimally used then the efficiency with which rock is broken is increased.
Higher gas pressure in drilled holes can be achieved by:    1. high density of an energetic substance;    2. high strength of an energetic substance;    3. efficient stemming and sealing of the gas produced in the hole; and    4. a high degree of coupling between the energetic substance and the hole.
The strength and density of the energetic substance in the hole relate to the relative energy per unit volume that is available for pressurising the hole.
Effective sealing of the energetic substance in the hole prevents the gas escaping in two ways.
The first is through the stemming column itself, which therefore relies on efficient stemming material and devices to prevent leakage through or dislodgement of the stemming column.
The second is through the fractures existing naturally in the rock or created by the drilling and breaking process. With existing non-explosive breaking methods the rock starts to fracture when pressurized by the gas, which results in the release of the gas through the fractures. Sometimes the early fracturing of the rock allows the gas to escape before the gas has built up sufficient pressure to displace the rock from its in-situ position, which then prevents the rock from being efficiently excavated.
Coupling is a very important property in achieving high pressures in a drilled hole as a tight interface between the energetic substance and the wall of the hole prevents gas pressure from being dissipated in any space that may exist between the two.
The sealing of the energetic substance in the drill hole and a tight coupling between the energetic substance and the confines of the hole are important factors in the achievement of a high-pressure environment within the drill hole.
Thus, if the gas can be retained in the hole until an optimal pressurization level has been reached and a tight coupling between the energetic substance and the confines of the hole is achieved, the available gas energy can be applied more efficiently to fracture and dislodge the rock in a controlled fashion. An object of the present invention is to achieve such a result.
The manner in which a cartridge is installed in a hole in a rock face, and the nature of the material surrounding the hole, play an important part in determining the efficiency with which the high pressure jet material, released upon ignition of the propellant, is utilised for fracturing the rock body. Stemming of any appropriate type is normally placed in the hole over the cartridge and is tamped down. The stemming acts to retain the cartridge in position when ignition of the propellant takes place. If the stemming is not adequately tamped or for any other reason is not in close contact with the cartridge then its restraining effect is reduced. A similar situation applies in respect of a lower end of the cartridge which, ideally, should be in intimate contact with a bottom of the hole.
In the radial sense the cartridge should be sufficiently small so that it can be inserted into the hole without undue effort. On the other hand the gap between an outer surface of the cartridge and an opposing surface of the wall of the hole should not be unduly large.
If a hole is formed in a rock mass which is partially fractured or fissured then the effectiveness of the energy, which is released upon ignition of the propellant in a cartridge, is reduced. This reduced effectiveness occurs for at least two reasons:
(a) firstly, the joints and fractures in the rock surfaces adjacent the cartridge allow the gas to be dissipated without directing the full amount of available energy into rock breaking; and
(b) secondly, the dissipation of the gas into the joints and fissures reduces the rate of pressurisation of the hole which in turn, as the burn rate is a positive function of the degree of confinement of the propellant, reduces the burn rate of the propellant and hence the rate at which gas is produced by the burning propellant.
The combination of the reduced rate of production of gas and the dissipation of the gas into the joints and fissures of the hole results in a reduced pressure environment in the hole which may be insufficient to break the rock.
Thus, if the gas can be retained in the cartridge until an optimal pressurisation level has been reached, the loss of effectiveness due to dissipation and reduced rate of gas production can be minimised.
Conversely, if the pressurisation of the cartridge is too high, the eventual release of the gas will cause the rockmass to break with resultant adverse side effects such as excessive flyrock, high levels of noise and increased overpressure or air blast effects.